Thermostat metal

ABSTRACT

A COMPOSITE THERMOSTAT METAL IS SHOWN TO COMPRISE A LAYER OF A SELECTED ALUMINUM ALLOY OF RELATIVELY HIGH COEFFICENT OF THERMAL EXPANSION WHICH IS IN A CONDITION OF TEMPER PROVIDING THE ALLOY WITH A YIELD STRENGTH OF AT LEAST ABOUT 30,000 POUNDS PER SQUARE INCH AT TEMPERATURES UP TO ABOUT 350* T., THE LAYER OF ALUMINUM ALLOY BEING METALLURGICALLY BONDED TO A LAYER OF METAL OF RELATIVELY LOW COEFFICIENT OF THERMAL EXPANSION.

Jan. 1, 5 5 TE IHERMOSTAT METAL Filed Nov. 56, 1971 3,782,908 THERMOSTAT METAL Sheldon S. White, Brookline, Mass, assignor to Texas Instruments Incorporated, Dallas, Tex. Filed Nov. 30, 1971, Ser. No. 203,266 Int. Cl. 133% 15/00 US. Cl. 29-1955 1 Claim ABSTRACT OF THE DISCLOSURE A composite thermostat metal is shown to comprise a layer of a selected aluminum alloy of relatively high coefficient of thermal expansion which is in a condition of temper providing the alloy with a yield strength of at least about 30,000 pounds per square inch at temperatures up to about 350 F., the layer of aluminum alloy being metallurgically bonded to a layer of metal of relatively low coeflicient of thermal expansion.

Composite thermostat metals embody metallurgically bonded layers of metal of relatively high and relatively low coefficients of thermal expansion. When such thermostat metals are subjected to increase in temperature, the different thermal expansion properties of these layer materials establish very high stresses in the themostat metal components and cause the metal to bend or flex in response to the temperature change, this flexing or bending being utilized in various conventional ways to actuate controls or the like in reaction to the temperature change. As will be understood, the degree of flexing which occurs in a thermostat metal in response to temperature (referred to as the flexivity of the thermostat metal) and the build up of stresses in the components of the thermostat metal are functions of the extent of the difference in the thermal expansion properties of the metals which are incorporated in the thermostat metal. For this reason, thermostat metals adapted to display high flexivities conventionally employ special metal alloys which are characterized by high strength and by relatively very high or relatively very low coeflicients of thermal expansion. These special alloys are expensive and the high costs associated with processing the special alloys tend to make high flexivity thermostat metals quite expensive.

It has now been recognized that there are a number of uses for high fiexivity thermostat metals Where the metals are not exposed to temperatures above about 350 F. during normal use. It has also been discovered that high flexivity thermostat metals adapted for use at temperatures up to this level can be manufactured at very low cost by employing selected aluminum alloys not previously used in thermostat metals to serve as relatively high thermal expansion components of the thermostat metals. In this regard, it is found that such aluminum alloys tend to display relatively quite high coefiicients of thermal expansion which can provide a thermostat metal with high flexivity. It has also been found that, although aluminum materials tend to display sharply lowering yield strength characteristics as the materials are heated even to relatively low temperatures, selected aluminum alloys can be placed in a condition of temper in which the alloys display quite substantial yield strengths at temperatures up to about 350 F. It has further been found that, even though these aluminum alloys display high coefficients of thermal expansion so that stresses tend to build up very rapidly in such alloys as the alloys are heated in a thermostat metal, the stresses which tend to build up in the aluminum alloys at temperatures up to about 350 F. even in high flexivity thermostat metals remain below the yield strengths which are achieved in the aluminum alloys in the noted condition of temper. As a result, the thermostat metals embody- United States Patent 3,782,908 Patented Jan. 1, 1974 "ice ing the selected aluminum alloys display stable, high flexivity, thermal response characteristics over a useful service life at temperatures up to about 350 F. and fulfill a need at substantially reduced cost.

It is an object of this invention to provide a novel and improved thermostat metal; to provide such a thermostat metal which is adapted to display high flexivity; to prov1de such a thermostat metal which is stable in its thermal response characteristics at temperatures up to about 350 F.; and to provide such a thermostat metal which is relatively quite inexpensive.

Other objects, advantages and details of the thermostat metal of this invention appear in the following more detailed description of preferred embodiments of the invention, the detailed description referring to the drawing in which FIG. 1 is a perspective view of a preferred embodiment of the thermostat metal of this invention.

Referring to the drawing, 10 in FIG. 1 illustrates the novel and improved thermostat metal of this invention which is shown to include a layer 12 of metal of relatively high coefiicient of thermal expansion and a layer 14 of metal of relatively low coefiicient of thermal expansion, these metal layers being metallurgically bonded together in any conventional way, preferably throughout the interface 16 between the metal layers.

In accordance with this invention, the layer 12 of the thermostat metal is formed of selected aluminum alloy which is in a condition of temper such that the aluminum material displays a yield strength of at least about 30,000 pounds per square inch at temperatures up to about 350 F, the aluminum alloy material being further characterized by a relatively high coeflicient of thermal expansion greater than about 1-0 microinches per inch per degree Fahrenheit. In this regard, an alloy is considered to be an aluminum alloy where the major constituent of the alloy is aluminum. There are a variety of commercially available aluminum alloys which can be provided with the desired condition of temper for this use in a number of conventional ways.

For example, in a preferred embodiment of this invention, the metal layer 12 is formed of an aluminum allo commonly identified as 2024 Aluminum Alloy (Ameri can Society for Metals Designation), this alloy having a nominal composition, by weight, of 4.5 percent copper, 1.5 percent magnesium, 0.6 percent manganese and the balance aluminum. In this preferred embodiment of this invention, the metal layer 14 is formed of the iron alloy commonly called Invar which has a nominal composition, by weight, of 36 percent nickel and the balance iron. Preferably, for example, the thermostat metal 10 has a total composite thickness of about 0.030 inch with each layer 12 and 14 of the composite forming approximately percent of the composite thickness. When these layer materials are cold roll bonded together in any conventional manner for metallurgically bonding the layers together along the interface 16 between the layers and when the aluminum material of the metal layer 12 is precipitation hardened in conventional manner to provide the aluminum material with T3 temper (Aluminum Standards Association Designation) the aluminum material displays a yield strength greater than about 30,000 pounds per square inch even when the aluminum material is heated to a temperature of about 350 F. In the range from room temperature up to about 350 F., this aluminum alloy material displays a very high coeflicient of thermal expansion of about 13.28 microinches per inch per degree Fahrenheit. As the material of the metal layer 14 displays a coefficient of thermal expansion of about 0.68 microinch per inch per degree Fahrenheit, the metal components of the thermostat metal 10 have coetlicients of thermal expansion which differ by approximately 12.60 microinches per inch per degree Fahrenheit and the preferred thermostat metal displays a high flexibility, determined in a conventional way, on the order of 183x10 Most important, this preferred thermostat metal 10 displays this high flexibility in applications where the temperatures to which the thermostat metal is exposed do not exceed 350 F. without being characterized by any significant change in thermal response characteristics of the thermostat metal over a usefully long service life. That is, despite the high stresses to which the aluminum material of the thermostat metal is subjected as a result of the high flexibility displayed by the thermostat metal, these stresses do not exceed the yield strengths of the aluminum material at the maximum temperature to which the thermostat metal is exposed, thereby assuring that the thermostat metal can be subjected to repeated temperature changes within the described range without undergoing any permanent deformation.

It will be understood that a variety of aluminum alloy materials are used in the metal layer 12 in accordance with this invention. For example, the metal layer can also embody 5052 Aluminum Alloy, 5056 Aluminum Alloy or 6061 Aluminum Alloy (ASM designations) within the scope of this invention, these alloys having nominal compositions, by weight, respectively as follows: 2.5 percent magnesium, 0.25 percent chromium, and the balance aluminum; 5.2 percent magnesium, 0.1 percent manganese, 0.1 percent chromium, and the balance aluminum; and 1.0 percent magnesium, 0.6 percent silicon, 0.25 percent copper, 0.25 percent chromium, and the balance aluminum. Each of these aluminum alloys displays a coeflicient of thermal expansion greater than about 10 microinches per inch per degree Fahrenheit and each of these aluminum alloys is readily provided with a condition of temper in a variety of conventional ways such that the alloy displays a yield strength of at least about 30,000 pounds per square inch at temperatures up to about 350 F.

It will also be understood that a variety of metals characterized by relatively lower coefiicients of thermal expansion are used in the metal layer 14 in accordance with this invention, although desirably the metal employed in the metal layer 12 is selected to display a coeflicient of thermal expansion which differs from the coefficient of thermal expansion of the aluminum alloy embodied in the metal layer 12 by at least about 8 microinches per inch per degree Fahrenheit. For example, in practical embodiments of this invention where one of the aluminum alloys specifically described above is utilized in the metal layer 12, the metal layer 14 embodies a metal alloy selected from the group consisting of alloys having the following nominal compositions, by weight: 38.65 percent nickel and the balance iron; 42 percent nickel and the balance iron; 45 percent nickel and the balance iron; 50 percent nickel and the balance iron; and 0.12 percent (max.) carbon, 1.00 percent (max.) manganese, 1.00 percent (max.) silicon, 14.00 to 18.00 percent chromium, and the balance iron. All of these materials display, or can be placed in a condition of temper by conventional means to display, yield strengths substantially greater than 30,000 pounds per square inch at temperatures up to 350 F. and display relatively lower coeflicients of thermal expansion than the aluminum alloy materials specifically described above. These materials are also adapted to be metallurgically bonded to the noted aluminum alloy materials in conventional ways.

It will also be understood that although the thermostat 4 metal 10 is illustrated as embodying only two metal layer 12 and 14, the thermostat metal 10 could also embody one or more intermediate metal layers which are located between the metal layers 12 and 14 and which are metallurgically bonded to the metal layers 12 and 14 within the scope of this invention. Further, the relative thicknesses of the metal layers 12 and 14 can also be varied from the substantially equal thicknesses shown in FIG. 1 within the scope of this invention. This invention includes all modifications and equivalents of the illustrated or described embodiments of this invention which fall within the scope of the appended claim.

I claim:

1. A composite thermostat metal comprising a plurality of layers of metal including one outer layer of metal formed of an aluminum alloy in a condition of temper displaying a yield strength of at least about 30,000 pounds per square inch at a temperature of about 350 Fahrenheit, said aluminum alloy being selected from the group consisting of an aluminum alloy consisting essentially of, by weight, 4.5 percent copper, 1.5 percent mag nesium, 0.6 percent manganese and the balance aluminum, an aluminum alloy consisting essentially of, by weight, 2.5 percent magnesium, 0.25 percent chromium, and the balance aluminum, an aluminum alloy consisting essentially of, by weight, 5.2 percent magnesium, 0.1 percent manganese, 0.1 percent chromium, and the balance aluminum, and an aluminum alloy consisting essentially of, by weight, 1.0 percent magnesium, 0.6 percent silicon, 0.25 percent copper, 0.25 percent chromium and the balance aluminum, said composite material further includingan opposite outer layer of metal having a coefiicient of thermal expansion lower than the coefiicient of thermal expansion of said aluminum alloy by at least about 8 microinches per inch per degree Fahrenheit and having a yield strength of at least about 30,000 pounds per square inch at a temperature of about 350 Fahrenheit, said opposite outer layer of metal being selected from the group consisting of an alloy consisting essentially of, by weight, 36 percent nickel and the balance iron, an alloy consisting essentially of, by weight, 38.65 perecnt nickel and the balance iron, an alloy consisting essentially of, by weight, 42 percent nickel and the balance iron, an alloy consisting essentially of, by weight, 45 percent nickel and the balance iron, an alloy consisting essentially of, by weight, 50 percent nickel and the balance iron, and an alloy consisting essentially of, by weight, 0.12 percent (max.) carbon, 1.00 percent (max.) manganese, 1.00 percent (max.) silicon, 14.00 to 18.00 percent chromium, and the balance iron, said layers of metal being metallurgically bonded together to display stable thermal response characteristics at temperatures up to about 350 Fahrenheit.

References Cited UNITED STATES PATENTS 2,240,824 5/1941 Alban 29--195.5 2,983,998 5/1961 Cherreau 29-l95.5 3,378,357 4/1968 Alban 29195.5 3,102,793 9/1963 Alban 291955 3,133,796 5/1964 Craig 29197.5 3,462,828 8/1969 Winter 29-l96.2 3,482,951 12/1969 Hubbell 29-l97.5

HYLAND BIZOT, Primary Examiner 

